The present invention relates to a multicolor image forming apparatus, and especially to a multicolor image forming apparatus, the circumferential length of the photoreceptor of which, being it is larger than the maximum document size, can be reduced.
For instance, when a conventional electrophotographic copier is used to form a multicolor image, processes such as charging, exposure, development, and transfer are repeated for each color and toner images of each color are transferred one another on a recording paper.
For example, an electrostatic latent image is formed at each process mentioned above by light, such a blue, green and red which is obtained through the color separation filters, and latent image is developed by yellow, magenta, cyan, and black toner. Thus the toner image of each color is formed on a photoreceptor surface and transferred to a recording paper, and repeating this process a multicolor image is formed on a recording paper.
But such a multicolor image forming method has several disadvantageous points which will be explained as follows. (1) The image must be transferred to a recording paper each time development is finished. For the reason, the size of the apparatus becomes large, and in addition it takes a long time to form an image. (2) When the image forming process is repeated, the positions of each color image tend to slip.
In order to improve this, there is a method for multicolor image forming, explained as follows. After a toner image is formed on a photoreceptor, furthermore a latent image is written on the photoreceptor with toner image and a toner image of different color is formed on the photoreceptor with former toner image, (in this case the toner images are superimposed, but each toner is not necessarily superimposed). This process is repeated. In other words, this multicolor image forming apparatus is the type in which each color toner is superimposed on the photoreceptor and transfer to a recording paper is conducted only once. This method can solve some of the above-mentioned problems. However, even by this method there are some problems such as disturbing the toner image obtained in a former stage at a subsequent developing stage or losing the color balance in the multicolor image because the toner in the former stage is mixed into that of the subsequent stage.
In order to avoid such defects, the following multicolor image forming method is proposed. The details of the method are as follows. The photoreceptor is kept out of contact with the developer layer which is to develop the electrostatic latent image formed on the photoreceptor, and the latent image is developed by toner in the developer which is cause to fly by the action of a D.C. bias current or a D.C. bias current superimposed with an A.C. component current.
According to the method, the images on the photoreceptor are not disturbed because the developer layer does not come into contact with the toner image formed beforehand.
The principle of this image forming method will be explained referring to the flow chart in FIG. 1.
FIG. 1 shows the variation of the photoreceptor surface potential. This is an example in which the electric polarity is positive.
PH represents the exposed portion of the photoreceptor. DA represents the non-exposed portion of the photoreceptor. DUP represents the raised electric potential caused by the positively charged toner T which adheres to the photoreceptor in the first development. CUP represents the raised electric potential of the exposed portion PH caused by the second charging. The photoreceptor is uniformly charged by a Scorotron charger and given a constant surface potential E. The surface potential is decreased to close to zero at the exposed portion PH by the first exposure conducted by an exposure source such as a laser, cathode-ray tube, a liquid crystal shutter, LED, or the like.
When a positive bias potential, whose D.C. component current is approximately equal to the surface potential E of the portion where exposure has not been conducted, is given to the developing apparatus, the positively charged toner T in the developing apparatus adheres to the exposed portion PH where the electric potential is relatively low. The first visible image is formed in this way.
The electric potential of the region where the visible image is formed rises by the amount of DUP because positively charged toner T adheres to the region. At the next stage, the second charging is carried out by the charger and electric potential further rises by the amount of CUP so that the initial surface potential E is obtained in this portion as well as the portion DA where exposure is not conducted.
Then, the second exposure is carried out on the photoreceptor surface where the surface potential E is uniform, and an electrostatic latent image is formed. After the same developing operation is conducted, the second visible image is obtained.
The multicolor toner image is obtained on the surface of the photoreceptor by repeating the process mentioned above. The multicolor image is obtained by transferring the toner image onto a recording paper and fixing it by heat or pressure.
The toner and electric charge which remain on the photoreceptor are cleared in preparation for the next multicolor image forming.
The discharge process by an exposure lamp or corona discharge may be conducted before every charging process.
The light source for exposure used to expose images may be either the same or different each time.
In the above-mentioned multicolor image forming method, toner images of 4 color, such as yellow, magenta, cyan, and black, are superimposed on the surface of the photoreceptor in many cases. The reason will be explained as follows.
A black image is theoretically obtained by superimposing 3 primary colors of yellow, magenta, and cyan. But a clear black image in accordance with requirements for letters and pictures is difficult to obtain by only 3 primary colors because the primary color toner for practical use does not have an ideal absorption wave length region and each toner images of 3 primary colors is difficult to register each other in proper position. In order to solve the problem, black is used in addition to the 3 primary colors, and 4 color toner images are superimposed to obtain a multicolor image similar to the document.
In the above-mentioned multicolor image forming method, a reverse developing method is used as an electrostatic latent image developing method.
In the reverse developing method, it is sufficient to expose only the portions on the photoreceptor where toner images are formed. It is not necessary in this method to expose the background, leaving unexposed spaces as in the normal developing method. Therefore, it is comparatively easy to form a latent image on a photoreceptor on which toner images have been already formed.
An advantageous point of this method is that the photoreceptors's life is lengthened. Furthermore, since electric charging after the second charging is conducted with the same polarity as the toner, no trouble is caused in electrostatic transfer.
As methods to form a latent image for multicolor image forming, two methods other than the above-mentioned method, in which the photoreceptor is uniformly charged and the electrostatic latent image is formed by exposure, are proposed by the inventors. They are the method in which the latent image is formed by charging a dielectric layer imagewise directly with a multineedle electrode and the method in which a magnetic latent image is formed by a magnetic head.
The above-mentioned methods are all able to express gradation. This type of gradation expression is what is called multistep gradation, and a large capacity for image data is needed.
In order to solve this problem, a new image data forming method is proposed which will be explained below. Each pixel is recorded in the binary system or the multiple system. The gradation is expressed in the matrix of each pixel. As a result, a large capacity for image data is unnecessary.
FIG. 8 is a schematic illustration which shows a three color image forming apparatus.
When the photoreceptor 51 makes one revolution, toner images of three colors of yellow, magenta, and cyan are superimposed.
In FIG. 8, the photoreceptor 51 is a light sensitive drum which is rotated in the arrowed direction.
As shown in FIG. 8, the scoroton charger 52, the exposure unit 53, and the developing unit 54 are arranged around the circumferential surface of the photoreceptor 51 in order to form yellow toner images. The scorotron charger 56, the exposure unit 57, and the developing unit 58 are serially arranged in order to form magenta toner images. Furthermore, the scorotron charger 60, the exposure unit 61, and the developing unit 62 are serially arranged in order to form cyan toner images. Accordingly, three color toner images are superimposed on the surface of the photoreceptor 51 when the photoreceptor makes one revolution. The numerals 55, 59, and 63 are developing sleeves. The numeral 64 is a recording paper feeding unit. The numeral 66 is a transfer electrode. The numeral 67 is a paper separating electrode. The numeral 68 is a fixing unit. The numeral 70 is a cleaing unit.
This apparatus can process rapidly, but it needs three pairs of charging, exposing, and developing units. As a result, the apparatus becomes large and its cost is increased. In order not to increase the cost of the apparatus and to make it compact, an image forming apparatus shown in FIG. 5 is proposed in which an exposure unit is used in common among three colors and each color image is superimposed by a plurality of revolutions of the photoreceptor.
The apparatus is used in the example of the present invention explained later. FIG. 5 is a schematic illustration of the multicolor image forming apparatus. FIG. 6 shows a laser unit which is applied to the image forming apparatus shown in FIG. 5. FIG. 7 shows a developing unit used in the image forming apparatus shown in FIG. 5.
In FIG. 5, the photoreceptor 11 rotates in the arrowed direction. It is uniformly charged by the scorotron charger 12. The surface of the photoreceptor which is uniformly charged, is exposed by an image exposure means to form an electrostatic latent image.
For instance, the laser unit 14 shown in FIG. 6 is used as the exposure unit and the photoreceptor is exposed to laser beam L to form electrostatic latent images corresponding to each color on the photoreceptor 11.
The electrostatic latent image corresponding to yellow is formed by being exposed to a laser beam modulated by the data relating to yellow.
The electrostatic laten image corresponding to yellow is developed by the first development unit 15 and the first toner image (yellow toner image) is formed on the photoreceptor 11.
Without this first toner image being transferred onto recording paper P, the photoreceptor 11 is charged by the scorotron charger 12 again.
Then, the laser beam is modulated by the data relating to magenta and the photoreceptor 11 is exposed to the modulated laser beam. The electrostatic latent image of magenta is formed in this way. This latent image is developed by the developing unit 16 and the second toner image (magenta toner image) is formed.
The latent image is developed by the third developing unit 17 in the same way. Then, the latent image is developed by the fourth developing unit. As a result, the third toner image (cyan toner image) and the fourth toner image (black toner image) are formed. The toner images are superimposed in order on the photoreceptor in this way and finally four color toner images are formed.
After the photoreceptor 11 is neutralized by the neutralizing lamp 40, these four color toner images are charged again by the charger 19 and transferred by the action of the transfer electrode 24 onto a recording paper P delivered from the paper feeding unit 20. The photoreceptor 11 is preferably charged by the charger 19 to the same electric potential as by the scorotron charger 12. By equalizing electric potential in this way, the condition of the toner which is charged again can become equal and toner image transfer can be conducted smoothly. It is desirable that the charger 19 is a scorotron charger.
The numeral 23 is a paper feeding roller. The numeral 22 is a guide plate. After recording paper P which holds the transferred toner image is separated by the paper separating electrode 25 from the photoreceptor 11, it is conveyed by the guide 26 and the conveyance belt 27 to the fixing rollers 28 and fixed by heat, then delivered to the paper delivery tray 29.
After toner images are transferred to recording paper P, the photoreceptor 11 is neutralized by the neutralizer 31 which was not used while toner images were being formed. Then the toner which remains on the surface of the photoreceptor is removed by the blade 32, a fur brush or a magnetic brush in the cleaning unit 30 to prepare for the next multicolor image forming.
However, in the above-mentioned image forming apparatus, the circumferential length of the photoreceptor (in the shape of drum or belt) is determined to be the length corresponding to the maximum image size, or the stroke length of the optical system for copying the maximum size document including the length corresponding to the back scanning of the optical system. Accordingly, the disadvantage of the apparatus is that the photoreceptor has a large portion which is not used when a small size image is formed on the photoreceptor.
In view of the fact mentioned above, a method is proposed in which the rotating speed of the photoreceptor is changed according to the image size. Refer to Japanese Patent Publication Open to Public Inspection No. 223857/1986. According to the above-mentioned proposal, the photoreceptor can be made compact, but the disadvantage of the proposal is that the printing speed slows down when a large size document is copied.